Resonant inelastic X-ray scattering in layered trimer iridate Ba4NbIr3O12: The density functional approach

We have investigated the electronic structure of Ba4NbIr3O12 within the density-functional theory (DFT) using the generalized gradient approximation while considering strong Coulomb correlations (GGA+U) in the framework of the fully relativistic spin-polarized Dirac linear muffin-tin orbital band-structure method. Ba4NbIr3O12 has a quasi-2D structure composed of corner-connected Ba3NbIr3O12 rimers containing three distorted face-sharing IrO6 octahedra. The Ir atoms are distributed over two symmetrically inequivalent sites: the center of the trimer (Ir1) and its two tips (Ir2). The Ir1- Ir2 distance within the trimer is quite small and equals to 2.547 A, at low temperature. As a result, there is clear formation of bonding and antibonding states. The large bonding-antibonding splitting stabilizes the dzz-orbital-dominant antibonding state of 5d holes and produces a wide energy gap at the Fermi level. The ground state of Ba4NbIr3O12 is a nonmagnetic singlet with relatively moderate spin-orbit coupling (SOC). We have theoretically calculated the x-ray absorption spectroscopy (XAS) spectra at the Ir L2, and Nb L3 edges as well as the photoemission spectrum of Ba4NbIr3O12. We have also presented a comprehensive investigation of the resonant inelastic x-ray scattering (RIXS) spectra at the Ir L3$ O K, Nb K, L3, M3, M5, and N3 edges. The RIXS spectrum of Ba4NbIr3O12 at the Ir L3 edge possesses several sharp features below 2 eV corresponding to transitions within the Ir t2g levels. The peak located at 3.2 eV is found to be due to t2g to eg transitions. The high energy fine structure above 5.3 eV is mostly determined by 5dO to tg and O2p to eg transitions. The spectral features between 8 and 12 eV are due to 5dO to eg transitions.